This invention relates generally to propulsion means for water vessels and more particularly to improvements in the water jet drive system for boats.
The majority of water vessels are driven by a prop shaft-driven propeller external to the hull of the boat. The majority of such systems dispose the propeller beneath the profile of the hull of the boat, thereby increasing the draft of the vessel and/or exposing the rotating propeller rendering it vulnerable to impact with debris, either submerged or at the surface of the water, and the water bottom.
One alternate to the exposed propeller-type system for propelling water vessels is the water jet drive system. This jet drive system includes a rigid impeller mounted for driven rotation within a housing having an inlet and an outlet. The inlet of the housing is disposed in the hull of the boat below the water line for water intake into the housing, while the housing outlet is disposed in the transom of the boat at or above the water line for high velocity discharge from the housing for propelling the boat. A prop shaft interconnects the impeller mounted within the housing to an engine or motor mounted within the boat, usually forward of the housing. By driven rotation, the impeller accelerates the water entering the housing, forcing the water to discharge from the housing outlet at the transom at a relatively high velocity.
One of the obvious benefits of the water jet drive system is that, by its very nature, the boat may be navigated in considerably shallower water than those driven by external propeller(s). This flush mounted profile offered by the jet drive system allows the operator to maneuver the boat over shallow sand bars, reefs, river rapids, beach heads and the like. However, in engaging in such shallow water operation, a considerable amount of abrasive material is likely to be drawn into the water jet drive housing, that abrasive material being held as sediment in the water and/or being drawn up from the water's bottom.
This abrasive material being drawn into the housing of the water jet and forced at very high rates of speed and pressure by, through and around the impeller results in excessive wear in both the impeller and the housing. The close tolerance relationship between the impeller and the housing, which is at the very heart of the performance of any jet drive, is jeopardized as a result of the abrasive action of the rapid forced passage of sediment therethrough.
Critical tolerances are established between the impeller and the housing so as to maximize the performance of the water jet drive system. When this close tolerance relationship is jeopardized by excessive wear, the performance and efficiency of the jet drive system are substantially compromised.
Housing-to-impeller clearance and alignments have been of great concern in this industry in an effort to provide an optimum balance between performance and service life of water jet drive systems. One early jet drive system incorporated an aluminum impeller within an aluminum housing. The close tolerances and alignment were controlled by shimming the axial positioning of the impeller within the housing to achieve clearance tolerances with the range of 0.2 to 0.3 mm. Because of the close tolerance requirements and the softness of the material, excessive wear resulted and usually required costly replacement of both housing and impeller.
One after market response to this wear deterioration dilemma resided in the replacement of the aluminum impellers with those fabricated of stainless steel and to replace the mating surface in the housing with a stainless steel liner. This after-marked "fix" has proved to be quite costly in that the stainless steel impellers are extremely expensive to fabricate and the stainless steel replacement ring within the housing requires the remachining of the worn housing to accept the stainless steel insert.
One manufacturer (Kawasaki) resdesigned the entire impeller-to-housing geometry to incorporate a tapered mating surface between the blade tips of the impeller and the housing which faciliated the axial shimming of the impeller to reachieve close tolerance between the two when wear conditions required same. However, the premature wear situation and reshimming the impeller is a costly and time consuming operation.
Some manufacturers have even resorted to building in excessive clearances to lessen the wear problem and alignment difficulties. However, it appears obvious that these manufacturers are substantially sacrificing performance and efficiency in favor of ease of alignment and extended service life.
The present invention provides an impeller having an annulur ring surrounding and integral or rigidly attached to the ends of the impeller blades. The ring has an outer circular surface which mates within a mating surface in the housing. However, close tolerance relationship between the outer surface of the ring and the housing need not be critical or maintained in order to retain high performance and efficiency of a water jet drive system incorporating the present invention. These results are obtained because any abrasive wear which occurs does so in relation to the inner surface of the ring which surface has virtually no bearing on performance or efficiency of the improved impeller.